Elsevier

Applied Surface Science

Volume 254, Issue 23, 30 September 2008, Pages 7821-7823
Applied Surface Science

Selective growth of stacked InAs quantum dots by using the templates formed by the Nano-Jet Probe

https://doi.org/10.1016/j.apsusc.2008.02.111Get rights and content

Abstract

We have demonstrated the selective area growth of stacked self-assembled InAs quantum dot (QD) arrays in the desired regions on a substrate and confirmed the photoluminescence (PL) emission exhibited by them at room temperature. These InAs QDs are fabricated by the use of a specially designed atomic force microscope cantilever referred to as the Nano-Jet Probe (NJP). By using the NJP, two-dimensional arrays with ordered In nano-dots are fabricated in the desired square regions on a GaAs substrate and directly converted into InAs QD arrays through the subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs are stacked. These stacked QDs exhibit the PL emission peak at a wavelength of 1.02 μm.

Introduction

The control of the nucleation site of quantum dots (QDs) is one of the key issues in the nanoscale design of optoelectronic functional QD devices [1], [2]. Recently, we have proposed a new nano-probe-assisted technique that enables the formation of site-controlled (SC) InAs QDs [3]. By using a specially designed atomic-force-microscope (AFM) cantilever referred to as the Nano-Jet Probe (NJP), two-dimensional (2D) arrays of ordered indium (In) nano-dots are reproducibly fabricated on a GaAs substrate [4]. These In nano-dots are directly converted into InAs arrays by the subsequent irradiation of arsenic flux using the droplet epitaxy technique [5], [6]. However, the quality of these fabricated InAs QDs is not very good since the photoluminescence (PL) emission from the QDs could not be obtained at room temperature. This is probably due to the following reasons. The first reason is the contamination of In, which is charged in the NJP. At present, In is charged in a separate vacuum system and the NJP is introduced into the QD fabrication system through air. Therefore, In, which is one of the constituents of InAs QDs, probably contains oxygen. Another possibility is the low-temperature process during the crystallization of the In nano-dots. Droplet epitaxy is a method for obtaining the crystals of compound semiconductors by annealing one of the melted materials in the vapor of the other materials at low temperatures, and it is attributed to the VLS mechanism [7]. Therefore, there is a possibility that the defects produced by this low-temperature technique result in a nonradiative center in the crystallized InAs QDs. However, the results mentioned in previous papers indicate that we obtained the InAs dots, although their quality is insufficient. Therefore, we decided to use the fabricated InAs QDs as a strain template for the stacking of the QDs. In this paper, we report on the results of the stacked SC-QDs fabricated by using the strain templates formed by the NJP method. The optical property of the fabricated stacked QDs is also reported.

Section snippets

Experimental

Fig. 1 shows the schematic illustrations of the experimental procedures. First, by using the NJP, we fabricated 2D In nano-dot arrays (Fig. 1(a)). Experiments involving the In nano-dot formation were performed using a conventional non-contact ultra-high vacuum (UHV)-AFM system operating at room temperature. The cantilever was a piezoelectric type with a hollow pyramidal tip made of silicon nitride, which has a micro-aperture at the apex and an In-reservoir tank within the stylus. By applying a

Results and discussions

We measured the optical property of the abovementioned stacked QD array. A selective grown area was excited using a He–Ne laser (λ = 633 nm) with an optical microscope; here, the size of the focal point on the sample was a few square microns and the excitation power was ∼0.5 mW. Fig. 3 shows the microprobe-PL mapping of the stacked InAs SC-SKQDs measured at room temperature. This image was obtained by plotting the output of an InGaAs CCD detector, which was accumulated in the wavelength range of

Summary

We have demonstrated the fabrication of the stacked InAs SC-SKQDs. Starting with the fabrication of the regular SC-QD arrays by the NJP method, we vertically aligned the SK-QDs by the strain-induced stacking method, thereby producing stacked SC-SKQD structures. The PL measurements reveal the good crystallographic quality of the stacked QD structures. Further improvements in the initial SC-QD array and growth conditions will increase the possibility of achieving a uniform QD size. This method

Acknowledgement

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) Project.

References (8)

  • S. Ohkouchi et al.

    Physica E

    (2004)
  • S. Ohkouchi et al.

    Thin Solid Films

    (2004)
  • S. Ohkouchi et al.

    Physica E

    (2008)
  • A. Imamoglu et al.

    Phys. Rev. Lett.

    (1999)
There are more references available in the full text version of this article.

Cited by (4)

View full text